The present invention relates in general to cordless telephone systems and, more particularly, to a frequency inversion scrambler in a cordless telephone.
Cordless telephones are widely used in residential and commercial environments because of their flexibility in allowing the user to venture some distance from the base unit during the conversation. The user speaks into the handset microphone and the voice data is transmitted by RF link to the base unit. The base unit sends the voice data across hard-wire telephone lines to the party on the other end. Voice data from the other party is also received by the base unit and transmitted over the RF link to the handset to complete the two-way conversation.
A common problem with many cordless telephones is the lack of security and privacy in the conversation. It is possible for non-parties to pick-up the same RF communication link, e.g., by way of another cordless telephone nearby or other RF receiver, and hear the private conversation. In the prior art, frequency inversion scramblers have been used to add privacy to the phone conversation by scrambling voice data transmitted over the RF link. Briefly, a frequency inversion scrambler inverts the transmitted frequency spectrum so that non-parties with standard cordless telephones cannot understand the conversation. The base unit for the handset inverts the frequency spectrum again to unscramble the voice data back to its original state for transmission over the telephone lines.
One prior art implementation of the frequency inversion scrambler has a low-pass filter receiving audio signals and a series capacitor between the low-pass filter and a modulator. The series capacitor blocks any DC offset in the audio signal as required for proper modulation. The modulation creates sum and difference frequencies about the modulation frequency. The modulated audio is again low-pass filtered to remove the modulation frequency and higher frequencies before transmission to the base unit. The remaining frequency spectrum is inverted with respect to the original audio signals and thus cannot be understood with standard cordless telephones. The base unit includes a similar low-pass filter and series capacitor into a modulator to re-invert the spectrum back to its original state. Another low-pass filter coupled to the output of the modulator removes the modulation frequency and higher frequencies and provides the base-band voice data for transmission across conventional telephone lines.
The low-pass filters in the frequency inversion scrambler are typically high order, say 10th-14th order, to provide the proper frequency response. The low-pass filter after the modulator needs to be high order for suppression of the modulation frequency and higher frequencies in order to isolate the inverted frequency spectrum. Unfortunately, a higher order filter also tends to increase group delay through the filter which reduces voice quality. It is impractical to move the modulation frequency farther from baseband and use a lower order low-pass filter because low frequency voice data would be lost due to the limited frequency operating band. In addition, the series capacitors are sufficiently large as to require external placement and associated IC pins.
Hence, a need exists for frequency inversion scrambler with lower order filters to reduce group delay while maintaining adequate suppression of the modulation frequency and higher frequencies.